Device and method for controlling air-fuel ratio of internal combustion engine

ABSTRACT

In a system for estimating an oxygen quantity stored in a catalytic converter of an internal combustion engine and feedback controlling a fuel injection quantity based on the estimated oxygen quantity, a detection value of the new intake air quantity is corrected according to an exhaust gas recirculation rate, and the oxygen quantity is estimated based on the corrected new intake air quantity.

FIELD OF THE INVENTION

[0001] The present invention relates to a device and method for controlling an air-fuel ratio of an internal combustion engine, in which an air-fuel ratio of the combustion mixture is controlled based on an oxygen quantity stored in a catalytic converter.

DESCRIPTION OF THE RELATED ART

[0002] Heretofore, there is known an air-fuel ratio control device that estimates an oxygen quantity stored in a catalytic converter based on an air-fuel ratio to be detected by an oxygen sensor disposed upstream of a catalytic converter and an exhaust gas quantity, to control an air fuel ratio of the combustion mixture so that the stored oxygen quantity reaches a target value (refer to Japanese Unexamined Patent Publication Nos. 6-249028,10-184425).

[0003] In estimating the above-mentioned stored oxygen quantity being stored, instead of directly measuring the exhaust gas quantity, a detection value of new intake air quantity is used, which is considered to substantially correspond to the exhaust gas quantity.

[0004] However, in an engine equipped with an exhaust gas recirculation system where a portion of exhaust gas is recirculated to an intake system, the recirculated exhaust gas is added to a new intake air quantity detected by an airflow meter to be flown into the intake system when the exhaust gas recirculation is performed.

[0005] Therefore, a difference is caused between the new intake air quantity detected by airflow meter and an actual exhaust gas quantity, and this difference leads to an estimation error in the stored oxygen quantity, deteriorating the control accuracy of the air-fuel ratio as a result.

SUMMARY OF THE INVENTION

[0006] Therefore, the present invention aims at enabling at a high accuracy the estimation of oxygen quantity stored in a catalytic converter even in a state where the exhaust gas recirculation is performed and thereby enabling to maintain the control accuracy of air-fuel ratio.

[0007] In order to achieve the above object, according to the present invention, a detected amount of new intake air quantity is corrected in accordance with a recirculation rate of exhaust gas, to estimate, based on the corrected result, an oxygen quantity stored in a catalytic converter.

[0008] These and other objects and features of the present invention will become understood from the following description with reference to the accompanying drawings.

BRIEF EXPLANATION OF THE DRAWINGS

[0009]FIG. 1 shows the system structure of an internal combustion engine; and

[0010]FIG. 2 is a block diagram showing the details of air-fuel ratio control according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0011]FIG. 1 is a diagram showing the system structure of an internal combustion engine according to an embodiment.

[0012] In FIG. 1, air is sucked into a combustion chamber of each cylinder in an internal combustion engine 1 mounted on a vehicle via an air cleaner 2, an intake passage 3, and an electronic controlled throttle valve 4 driven to open/close by a motor.

[0013] An electromagnetic fuel injection valve 5 is further disposed to inject fuel directly into the combustion chamber of each cylinder, and an air-fuel mixture is formed within the combustion chamber by the fuel injected by fuel injection valve 5 and the air sucked into the combustion chamber.

[0014] Fuel injection valve 5 is supplied with the power to a solenoid thereof by an injection pulse signal output from a control unit 20, to inject fuel adjusted to a predetermined pressure.

[0015] The air-fuel mixture formed within the combustion chamber is ignited by an ignition plug 6 to be combusted.

[0016] However, internal combustion engine 1 is not limited to the aforementioned direct injection type gasoline engine, but may be an internal combustion engine where fuel is injected into an intake port.

[0017] The exhaust gas of engine 1 is discharged through an exhaust passage 7 that is disposed with a catalytic converter 8 for purifying the exhaust gas.

[0018] Catalytic converter 8 is a three-way catalytic converter having the oxygen storing ability, to oxidize carbon monoxide CO, hydrocarbon HC and to reduce nitric oxide NOx, being the three harmful components included in the exhaust gas, thereby converting them into harmless carbon dioxide, water vapor and nitrogen.

[0019] The purification performance of three-way catalytic converter 8 is the greatest when an air-fuel ratio is a stoichiometric air-fuel ratio. When the air-fuel ratio is lean and the oxygen quantity is excessive, the oxidizing action becomes active but the reducing action becomes inactive, and in reverse, when the air-fuel ratio is rich and the oxygen quantity is low, the oxidizing action becomes inactive but the reducing action becomes active.

[0020] However, since three-way catalytic converter 8 has the oxygen storing ability, when the air-fuel ratio becomes temporarily rich, the oxygen stored in catalytic converter 8 is used, and when the air-fuel ratio becomes temporarily lean, the excessive oxygen is stored in catalytic converter 8 so that the exhaust gas purification performance can be maintained.

[0021] Accordingly, in order to be able to reduce nitric oxide NOx when the air-fuel ratio becomes leaner than the stoichiometric air-fuel ratio and also to oxidize carbon monoxide CO and hydrocarbon HC when the air-fuel ratio becomes richer than the stoichiometric air-fuel ratio, it is required to maintain the oxygen quantity to be stored in three-way catalytic converter 8 to approximately half the maximum storage amount, so that excessive oxygen can be stored to the catalytic converter when needed and so that oxygen necessary for the oxidation process can be supplied by eliminating from the stored oxygen when needed.

[0022] Therefore, control unit 20 mentioned above estimates the stored oxygen quantity in three-way catalytic converter 8 within an operation region where a target air-fuel ratio is the stoichiometric air-fuel ratio, and feedback controls a fuel injection quantity of fuel injection valve 5 so that when the estimated stored oxygen quantity is smaller than a target quantity, the air-fuel ratio is shifted to lean so as to increase the stored oxygen quantity, whereas when the estimated stored oxygen quantity is greater than the target quantity, the air-fuel ratio is shifted to rich so as to eliminate the excessive oxygen thereby reducing the stored oxygen quantity.

[0023] Further, engine 1 is provided with an exhaust gas recirculation (EGR) device where a portion of the exhaust gas is recirculated to the intake side.

[0024] The exhaust gas recirculation device mentioned above comprises an exhaust gas recirculation passage 18 connecting an exhaust manifold 16 and an intake collector 17, and an EGR control valve 19 disposed in exhaust gas recirculation passage 18. When EGR control valve 19 is controlled to open, due to a differential pressure generated between the front and back of EGR control valve 19, a portion of the exhaust gas is recirculated to intake collector 17.

[0025] Control unit 20 incorporates therein a microcomputer including CPU, ROM, RAM, A/D converter, input/output interface and the like. When receiving output signals from various sensors, control unit 20 performs an operation process based on these signals, to control the opening of electronic controlled throttle valve 4, the injection quantity and injection timing of fuel injection valve 5, the ignition timing of ignition plug 6, and also to control an exhaust gas recirculation rate through the control of EGR control valve 19.

[0026] In the control of exhaust gas recirculation rate, control unit 20 sets a target exhaust gas recirculation rate based on operating conditions such as an engine load and an engine rotation speed, to output a control signal corresponding to the target exhaust gas recirculation rate, to an actuator driving EGR control valve 19 such as a step motor or an electromagnetic coil.

[0027] As various sensors, there is provided a crank angle sensor 21 that detects a crank angle of engine 1 and a cam sensor 22 that takes out cylinder discrimination signals from a camshaft, and based on a signal from crank angle sensor 21, the engine rotation speed Ne is computed.

[0028] Other than the above, there is provided an airflow meter 23 that detects a new intake air quantity Q at the upstream of throttle valve 4 in intake passage 3, an accelerator sensor 24 that detects a depression quantity APS of an accelerator pedal, a throttle sensor 25 that detects the opening TVO of throttle valve 4, a water temperature sensor 26 that detects the cooling water temperature Tw of engine 1, an oxygen sensor 27 that detects in wide range an oxygen concentration within the exhaust gas, and a vehicle speed sensor 28 that detects the vehicle speed VSP.

[0029] Now, the air-fuel ratio control of control unit 20 based on the stored oxygen quantity is explained with reference to a block diagram of FIG. 2.

[0030] In the block diagram of FIG. 2, at a correction coefficient setting unit 101, a correction coefficient for correcting the new intake air quantity Q detected by airflow meter 23 is set based on the exhaust gas recirculation rate at that time.

[0031] The correction coefficient is set to 1.0 when the exhaust gas recirculation rate is 0, in other words when the exhaust gas recirculation is stopped, and further set to a greater value as the exhaust gas recirculation rate increases.

[0032] The correction coefficient is set by either searching for a table storing in advance the correction coefficient corresponding to the exhaust gas recirculation rate, or through an operation using the exhaust gas recirculation rate as a variable.

[0033] Airflow meter 23 detects the new intake air quantity of the engine. However, when the exhaust gas recirculation is performed, the recirculated exhaust gas is flown with the new air flowing into the engine, and an exhaust gas quantity will correspond to the total amount of the new intake air quantity and a recirculated exhaust gas quantity.

[0034] Accordingly, in a state where the exhaust gas recirculation is being performed, if the stored oxygen quantity is estimated based on an assumption that the new intake air quantity detected by airflow meter 23 indicates the exhaust gas quantity, the oxygen quantity is judged based on a quantity smaller than an actual exhaust gas quantity to estimate the storage oxygen quantity based on the judged oxygen quantity, causing degradation of estimation accuracy of the stored oxygen quantity.

[0035] Therefore, in the present embodiment, in order to estimate the stored oxygen quantity according to the actual exhaust gas quantity, the detection value of the new air intake quantity is increasingly corrected by an amount of recirculated exhaust gas, so that the corrected new intake air quantity indicates the actual exhaust gas quantity correctly, to be used in the estimating operation of the stored oxygen quantity.

[0036] In other words, the value corrected by the correction coefficient corresponds to the exhaust gas quantity including the amount of the recirculated exhaust gas.

[0037] The correction coefficient set by correction coefficient setting unit 101 is multiplied on the new intake air quantity Q detected by airflow meter 23, and data of the new intake air quantity Q after the multiplication correction is further multiplied by a deviation Δλ between the stoichiometric air-fuel ratio and the air-fuel ratio detected by oxygen sensor 27.

[0038] In the present embodiment, an excess air ratio showing the air-fuel ratio based on oxygen sensor 27 is computed, and since the excess air ratio corresponding to the stoichiometric air-fuel ratio is 1, the deviation Δλ is computed as:

Δλ=actual excess air ratio −1

[0039] If the air-fuel ratio of the combustion air-fuel mixture is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio deviation Δλ becomes a positive value, while if it is richer, the deviation Δλ becomes a negative value, which correspond to the situation where the stored oxygen quantity of catalytic converter 8 is increasingly changed when the air-fuel ratio of the combustion air-fuel mixture is leaner than the stoichiometric air-fuel ratio, while the stored oxygen quantity of catalytic converter 8 is decreasingly changed when the air-fuel ratio of the combustion air-fuel mixture is richer than the stoichiometric air-fuel ratio.

[0040] A constant K is multiplied on the multiplication result of the new intake air quantity Q and the air-fuel ratio deviation Δλ, and this multiplication result is integrated one after another by an integrator 102, to thereby obtain the stored oxygen quantity of catalytic converter 8.

[0041] Here, the new intake air quantity Q utilized in estimating the stored oxygen quantity is corrected to a value including the amount of recirculated exhaust gas during the exhaust gas recirculation state, so the stored oxygen quantity can be obtained based on the actual exhaust gas quantity even the exhaust gas recirculation is performed.

[0042] Next, a deviation between the estimated value of the stored oxygen quantity output from integrator 102 and the target value corresponding to approximately half the maximum storage oxygen quantity is computed.

[0043] Then, at a feedback correction coefficient setting unit 103 input with data of deviation of the stored oxygen quantity, a feedback correction coefficient of the air-fuel ratio is computed so that the estimated value of the stored oxygen quantity corresponds to the target value.

[0044] That is, the feedback correction coefficient is set so that when the stored oxygen quantity is smaller than the target quantity, the air-fuel ratio is shifted to leaner so as to increase the stored oxygen quantity, and in contrast, when the stored oxygen quantity is greater than the target quantity, the air-fuel ratio is shifted to richer so as to eliminate the excessive oxygen thereby decreasing the stored oxygen quantity.

[0045] At an injection quantity computing unit 104, the above feedback correction coefficient is utilized, to correct a basic fuel injection quantity thereby computing a final fuel injection quantity, and an injection pulse signal corresponding to the computed final fuel injection quantity is output to fuel injection valve 5.

[0046] The entire contents of japanese patent application no. 2000-365226 filed Nov. 30, 2000 are incorporated herein by reference: 

What is claimed:
 1. An air-fuel ratio control device of an internal combustion engine comprising: a fuel injection valve that injects fuel into said engine; a catalytic converter disposed to an exhaust pipe of said engine; an intake air quantity sensor that detects a new intake air quantity of said engine; an oxygen sensor, disposed to said exhaust pipe on the upstream side of said catalytic converter, that detects an oxygen concentration of the exhaust; an exhaust gas recirculation pipe that recirculates a portion of the exhaust gas of said engine into an intake pipe; an exhaust gas recirculation rate detecting unit that detects a recirculation rate of the exhaust gas being recirculated via said exhaust gas recirculation pipe; a new intake air quantity correcting unit that corrects the detection value of said new intake air quantity based on said exhaust gas recirculation rate; an oxygen quantity estimating unit that estimates an oxygen quantity stored in said catalytic converter based on said oxygen concentration and said detection value of the new intake air quantity corrected based on the exhaust gas recirculation rate; and a feedback control unit that feedback controls a fuel injection quantity of said fuel injection valve so that said oxygen quantity approximates a target quantity.
 2. An air-fuel ratio control device of an internal combustion engine according to claim 1, wherein said new intake air quantity correcting unit corrects the detection value of said new intake air quantity to a higher value as the higher the exhaust gas recirculation rate is.
 3. An air-fuel ratio control device of an internal combustion engine according to claim 1, wherein said new intake air quantity correcting unit comprises: a correction coefficient setting unit that sets a correction coefficient according to said exhaust gas recirculation rate; and a correction computing unit that multiplies said correction coefficient on the detection value of said new intake air quantity to output the multiplied result.
 4. An air-fuel ratio control device of an internal combustion engine according to claim 1, wherein said oxygen quantity estimating unit comprises: an air-fuel ratio deviation computing unit that computes a deviation between a stoichiometric air-fuel ratio and an air-fuel ratio corresponding to said oxygen concentration; and an estimating unit that estimates the oxygen quantity stored in said catalytic converter based on said air-fuel ratio deviation and said corrected detection value of the new intake air quantity.
 5. An air-fuel ratio control device of an internal combustion engine according to claim 1, wherein said oxygen quantity estimating unit comprises: an air-fuel ratio deviation computing unit that computes a deviation between a stoichiometric air-fuel ratio and an air-fuel ratio corresponding to said oxygen concentration; a first multiplying unit that multiplies said air-fuel ratio deviation and said corrected detection value of the new intake air quantity; a second multiplying unit that multiplies the multiplied result of said first multiplying unit by a constant; and an integrating unit that integrates the multiplied result of said second multiplying unit.
 6. An air-fuel ratio control device of an internal combustion engine according to claim 1, wherein said feedback control unit comprises; a target setting unit that sets half the oxygen quantity of the maximum oxygen quantity capable of being stored in said catalytic converter as said target value.
 7. An air-fuel ratio control device of an internal combustion engine according to claim 1, wherein said feedback control unit comprises: an oxygen quantity judging unit that judges whether said estimated oxygen quantity is greater than said target value or not; a decremental correcting unit that decreasingly corrects said fuel injection quantity when the estimated oxygen quantity is smaller than said target value; and an incremental correcting unit that increasingly corrects said fuel injection quantity when said estimated oxygen quantity is greater than said target value.
 8. An air-fuel ratio control device of an internal combustion engine comprising: a fuel injection valve that injects fuel into said engine; a catalytic converter disposed to an exhaust pipe of said engine; an intake air quantity sensor that detects a new intake air quantity of said engine; an oxygen sensor, disposed to said exhaust pipe on the upstream side of said catalytic converter, that detects an oxygen concentration of the exhaust; an exhaust gas recirculation pipe that recirculates a portion of the exhaust gas of said engine into an intake pipe; exhaust gas recirculation rate detecting means for detecting a recirculation rate of the exhaust gas being recirculated via said exhaust gas recirculation pipe; new intake air quantity correcting means for correcting the detection value of said new intake air quantity based on said exhaust gas recirculation rate; oxygen quantity estimating means for estimating an oxygen quantity stored in said catalytic converter based on said oxygen concentration and said detection value of the new intake air quantity corrected based on the exhaust gas recirculation rate; and feedback control means for feedback controlling a fuel injection quantity of said fuel injection valve so that said oxygen quantity approximates a target quantity.
 9. An air-fuel ratio control method of an internal combustion engine comprising the steps of: detecting a new intake air quantity of said engine; detecting an oxygen concentration of the exhaust at the upstream side of a catalytic converter disposed to an exhaust pipe of said engine; detecting a recirculation rate of the exhaust gas with respect to said engine; correcting the detection value of said new intake air quantity based on said exhaust gas recirculation rate; estimating an oxygen quantity stored in said catalytic converter based on said oxygen concentration and said detection value of the new intake air quantity corrected based on the exhaust gas recirculation rate; and feedback controlling a fuel injection quantity of said fuel injection valve so that said oxygen quantity approximates a target quantity.
 10. An air-fuel ratio control method of an internal combustion engine according to claim 9, wherein said step of correcting a new intake air quantity comprises the step of; correcting the detection value of said new intake air quantity to a higher value as the higher the exhaust gas recirculation rate is.
 11. An air-fuel ratio control method of an internal combustion engine according to claim 9, wherein said step of correcting a new intake air quantity comprises the steps of: setting a correction coefficient according to said exhaust gas recirculation rate; and multiplying said correction coefficient on the detection value of said new intake air quantity to output the multiplied result.
 12. An air-fuel ratio control method of an internal combustion engine according to claim 9, wherein said step of estimating an oxygen quantity comprises the steps of: computing a deviation between a stoichiometric air-fuel ratio and an air-fuel ratio corresponding to said oxygen concentration; and estimating the oxygen quantity stored in said catalytic converter based on said air-fuel ratio deviation and said corrected detection value of the new intake air quantity.
 13. An air-fuel ratio control method of an internal combustion engine according to claim 9, wherein said step of estimating an oxygen quantity comprises the steps of: computing a deviation between a stoichiometric air-fuel ratio and an air-fuel ratio corresponding to said oxygen concentration; multiplying said air-fuel ratio deviation and said corrected detection value of the new intake air quantity; multiplying the multiplied result of said first multiplying unit by a constant; and integrating the multiplied result of said second multiplying unit.
 14. An air-fuel ratio control method of an internal combustion engine according to claim 9, wherein said step of feedback controlling a fuel injection quantity comprises the step of; setting half the oxygen quantity of the maximum oxygen quantity capable of being stored in said catalytic converter as said target value. 